a. Field of the Invention
This invention relates to self-supporting, mouldable, vibration damping sheets, to methods of manufacturing them and to vibration damped systems utilizing such sheets.
B. Description of Prior Art
It is well known that noise emanates from a structure excited to resonant vibration. An example would be the noise produced by a gong after it has been struck by a hammer. The amount of this noise can be drastically reduced by the application of a layer of a vibration damping compound to the surface of the structure subjected to excitation.
Vibration damping compounds are frequently applied to such structures as automobile doors, electric typewriter housings, air conditioning ducts, motor housings, grinder housings, and air-compressor housings.
Various methods of applying the damping compounds have been employed. If the compound is in the form of a sheet it may be adhered to the structure, if in the form of a putty it may be trowelled on, and if in the form of a paint it may be sprayed or painted on.
These are two types of vibration damping. If there is a layer of stiff, rigid material adhered to the outer surface of the damping compound, "constrained layer" damping is said to occur. If there is no such constraining layer, the damping is referred to as "free-layer" or "extensional" damping.
Many different compounds have been used as free layer vibration dampers. Some are based on rubber or other elastomers, while others are based on asphalt, and others, on thermoplastic or thermosetting polymers. Such materials have not generally been satisfactory because on the basis of a given thickness or weight their damping effectiveness has been low and they often dampen vibration over only a narrow range of frequency and temperature. Furthermore, for certain applications many of them do not possess the necessary stability to air, water, heat, light, fuels, lubricants, abrasion, impact, etc. Very few of them can be manufactured as a self-supporting sheet and still fewer are capable of being produced as a sheet which can subsequently be moulded to a substrate. Some which are in sheet form are difficult to adhere well to the vibrating structure. Others are so rigid that they cannot be adhered to a curved surface.
Particularly effective free-layer vibration damping compounds have been based on thermoplastic polymers. For example, U.S. Pat. No. 3,399,103 describes the use of a copolymer of ethylene and vinyl chloride; U.S. Pat. No. 3,489,242 describes the use of polyurethane, and ethylene/vinyl acetate copolymer and others, and U.S. Pat. No. 3,271,188 refers to polymer mixtures comprising at least three polymers which differ in their second order transition temperature.
A vibration damping product which is used commercially on a large scale is made by mixing a filler of mica and clay with an emulsion of polyvinyl acetate containing some plasticizer, and then spreading the resulting aqueous mixture onto a backing to form a coating and drying it in an oven to drive off the water. This vibration damping sheet and the method of manufacturing it suffer from several disadvantages. The use of an emulsion of polyvinylacetate necessitates the use of an emulsifier which is retained in the coating; this emulsifier lowers the water resistance of the final coating to an extent which is not good. The drying operation must be carried out slowly, otherwise blisters form at the surface of the coating producing a commercially unacceptable product. Thus, the production time is lengthened and careful monitoring is required to assure an acceptable product.
A further disadvantage of this existing process is that it requires the use of a backing, which may be, for example, a textile fabric; this backing which is unnecessary acoustically is expensive and increases the cost of the final product. Furthermore, the backing renders the final vibration damping product non-mouldable.
Another serious disadvantage in this existing process is that the amount of filler that can be added, for a given total solids content, is limited by the maximum viscosity permisible to allow spreading or spraying of the aqueous coating mixture on the backing. On a dry basis, the maximum filler content is approximately 60% by weight when mica and clay are used. This is a serious drawback because if more filler could be added the vibration damping product would be cheaper, and as we have now discovered the damping would be greater and would also be effective over a broader temperature range.